A conventional power supply, referring to FIG. 1, includes an EMI filter unit 2, a commutation unit 3, a power factor correction circuit 4, a standby power system 5, a pulse width modulation (PWM) unit 6 and a main power system 7. A first switch unit 11 is provided to control ON/OFF between the power supply and an AC power input source 9. The standby power system 5 is connected to a second switch unit 12 to control operation of the PWM unit 6. The main power system 7 has at least one high voltage output end (shown by +12V in FIG. 1) and at least one low voltage output end (shown by +5V and +3.3V in FIG. 1). However, the standby power system and the main power system do not adopt a modular power output structure. To provide different output watts design of element parameters of the standby power system or main power system has to be done anew. And there is no expandability. To improve the design of power supply and compatibility of the power supply of the same series, R.O.C. Patent No. 1260848 entitled “Chain control circuit for parallel power supply” includes a plurality of parallel power supplies connecting to a control unit through a power supply switch unit. The power supply switch unit issues signals so that the control unit orders the parallel power supplies to deliver output at the same time. The parallel power supplies also can output rated watts as deigned. Such a design can accumulate output watts of the power supplies to meet requirements. The number of the parallel power supplies can be increased or decreased to alter the output watts. Hence design of the power supply is more flexible and can be done faster. Repairs and maintenance also are simpler. However, ON/OFF of the power supplies are done at the same time under the order of the control unit. When the parallel power supplies are started simultaneously, an inrush current is generated that might cause damage of computer elements or trigger a protection mechanism to make the computer elements not able to start normally. There is still a need to control output of a plurality of power supplies coupled in parallel.